T=distance over speed 
T=40m over 9.8ms 
T=answer
        
             
        
        
        
The answer is:
B. <span>X: Work is done to the system and temperature increases.
Y: Work is done by the system and temperature decreases.</span>
        
                    
             
        
        
        
Then every line segment has one and only one mid-point.
(っ^▿^)
        
                    
             
        
        
        
The answer is 100mm/s. I hope this helps :)